1. Field of the Invention
The present invention is generally concerned with devices used in aircraft to start a gas turbine.
It is more particularly concerned with the situation in which the turbine to be started is intended in particular to serve as an auxiliary electrical power unit commonly called an APU.
Even more particularly, the present invention concerns the situation in which, in the case of an aircraft APU, the turbine is a high-power turbine.
2. Description of the Prior Art
At present the turbine to be started is usually at the rear of the aircraft, to be more precise in the tail of the aircraft.
Its starting sequence entails it being driven mechanically until its speed has reached a value substantially equal to 50% of its nominal value in normal operation.
As soon as its speed thereafter reaches 95% of its nominal value the turbine is coupled to a synchronous generator which, controlled accordingly, then operates as an alternator and generates the electrical power required to supply the onboard equipment of the aircraft.
At present two different technologies are routinely used for starting a gas turbine of the above kind.
In accordance with the first, and older, of these starting technologies the starter device includes an independent drive electric motor, the output shaft of which can be mechanically coupled to the input shaft of the turbine to be started and supplied with direct current from the onboard batteries of the aircraft.
As used above, xe2x80x9cindependentxe2x80x9d drive electric motor means an electric motor separate from the alternator that is coupled to the turbine.
In practice the batteries that power the independent drive electric motor are usually in the front of the aircraft, in the cargo hold.
The starting torque required to start a high-power turbine and the corresponding starting power are necessarily very high.
Under these conditions the starting current to be supplied to the motor can itself be very high, frequently greater than, 1,000 A, and difficult to control.
It is impossible, under these conditions, to use the batteries in the front of the aircraft, if for no other reason than the resulting heat dissipation in the electrical conductors connecting the batteries to the motor.
Batteries exclusively used to power the starter motor must be provided at the rear of the aircraft, as close as possible to the motor.
This leads to an unwanted and non-negligible increase in all up weight, in the order of 50 kg.
In the second existing starting technology, usually called the alternator-starter technology, which is currently tending to predominate because it is normally able to achieve better performance, the synchronous generator coupled to the turbine is controlled so as to operate as a motor and applies to the turbine the mechanical power needed to start it.
Because an independent starter motor is eliminated and because the electrical conductors required to connect it are also eliminated the weight and the dimensions of the system are advantageously limited.
However, the synchronous generator is specifically designed to operate normally as an alternator and does not offer good performance when operated as a starter motor.
In particular, its torque constant is low.
Starting a high-power turbine requires a high current to be passed through its armature and it is therefore necessary to control the synchronous generator using a complex and heavy converter and to provide a second converter to control it when operated as a motor, which inevitably imposes a penalty in terms of all up weight.
For high values of the starting current to be supplied the weight and size of such converters can be prohibitive in the case of an aircraft.
Finally, modern high-power turbines often have two separate shafts, one for starting them and the other for driving the synchronous generator, and the two shafts are thermodynamically coupled together in a non-reversible fashion with the result that it is not possible to operate the synchronous generator as a motor.
The present invention consists is a device for starting a gas turbine in an aircraft which, while being suitable for starting a high-power turbine without imposing any constraints on the other onboard equipment of the aircraft, avoids the drawbacks referred to above and additionally has other advantages.
The invention is a device for starting a gas turbine in an aircraft including an electrical power supply device adapted to deliver a direct current and a drive electric motor connected to the power supply device, independent of the turbine to be started and having an output shaft adapted to be mechanically coupled to an input shaft of the turbine to rotate with it, wherein the drive electric motor is a three-phase alternating current synchronous motor and is controlled by an inverter connected to the power supply device.
If the power supply device employed includes one or more storage batteries it is advantageously possible to use the storage batteries usually available at the front of the aircraft.
In accordance with the invention, all that is required is to provide, at the same location, a voltage converter adapted to raise the voltage to a value sufficient for the heat dissipation in the electrical conductors that are then inevitably used between the front part and the rear part where the turbine to be started is located to remain within acceptable limits for the same power.
The same applies if, as an alternative, the electrical power supply device employed includes the three-phase alternating current supply usually present on an aircraft.
In accordance with the invention, if the onboard three-phase alternating current supply is used, all that is required is to connect it via a current converter.
In either case, a non-negligible weight saving is obtained and it is worth emphasizing that, in accordance with the invention, this result is achieved in a someone paradoxical manner by dissociating two devices, here the independent motor used for starting and the synchronous generator used in normal operation, and thereby reverting to the technology of the independent drive electric motor, whereas in the alternator-starter technology, and in accordance with an attitude that is widespread in the aeronautical field, a weight saving is usually obtained by integrating separate devices.
Be this as it may, using an independent drive motor for starting has the advantage that the best possible use is made in service of the properties of the synchronous generator coupled to the turbine without the design of the synchronous generator needing to allow for operation in motor mode which limits the risk of failure or of degradation of performance to which any malfunction could lead during starting in motor mode.
Controlling the drive motor with an inverter, preferably using the vector control technique, has the advantage of optimum control of the supply current and therefore of the torque that it delivers so that the system can be adapted optimally to suit the parameters of the turbine, with the benefit of optimizing its operating conditions and of making the system more reliable.
In particular, the service life of the turbine is advantageously increased.
In accordance with the invention the phase shift between the current supplied to the motor and the electromotive force generated by the motor is controlled in accordance with the operating point of the motor.
This phase shift control can be independent of the vector control, but, if associated therewith, it has the advantage of optimizing the performance of the motor.
Moreover, the inverter and, in particular, the phase and/or the amplitude of its output current and therefore the drive torque supplied, can be controlled in accordance with measured instantaneous parameters of the turbine such as the exhaust gas temperature, for example, and/or in accordance with preprogrammed parameters, such as the flight phase or the outside temperature, for example.
The torque supplied by the motor can therefore advantageously be adapted to suit the needs of the moment even more closely.
In a preferred embodiment of the invention the voltage converter downstream of the electrical power supply device is made up of a plurality of individual converter modules which has the advantage of reducing the size of the system while allowing a high power to be achieved.
Also in a preferred embodiment of the invention the various converter modules employed, the inputs of which are connected in parallel, have their outputs connected in series.
The resulting reduction in their individual conversion ratio advantageously overcomes technical problems that could otherwise result from their implementation.
Further features and advantages of the invention will emerge from the following description given by way of example with reference to the accompanying diagrammatic drawings.